This invention is particularly concerned with military aircraft as opposed to civilian or commercial aircraft and even more particularly to the class of aircraft that would fall in the fighter class. As is well known, fighter aircraft as presently perceived are designed for the combat box within the flight spectrum. Engine operation at 0.9 Mach No. at an altitude of 15,000 feet is representative of the combat box. Because of the nature of its flight mission this class of aircraft typically undergoes rather violent maneuvers, calling for much manipulation of the power lever to change thrust of the engine so as to accelerate and decelerate at very severe conditions. When the aircraft undergoes these maneuvers the pilot will typically exercise power lever movements called bodies, chops, snaps and the like which result in engine speed, temperature, and airflow excursions. Under such maneuver excursions the rotor speeds of the fan and high pressure compressor rotors will vary from a high level of the operating spectrum called intermediate power to a lower level called part power or idle power. While these rotor speeds and thrusts are varying during maneuver excursions, the variable geometry parts of the engine are also moving. Fan and high compressor vane angles are changing with rotor speed and the variable area exhaust nozzle is changing position, typically +5% to +10% from the intermediate power area as the engine moves toward idle power, decelerates. Exhaust nozzle area normally closes 5% to 10% as the engine accelerates.
To more fully understand the background of this technology the graphs in FIG. 1 show a typical schedule of a military aircraft engine designed to operate at 0.9 Mach No. at an altitude of 15,000 feet. The graphs describe the speed of the high compressor and fan pressure rotors, (N2 and N1 respectively), the inlet temperature of the turbine, the total airflow at the fan inlet (Wat2) and the area of the exhaust nozzle (Aj) at various thrust levels. These parameters are typical in a single spool or multiple spool axial flow turbine power plant either of the straight jet or fan jet configuration; the latter, for example, being exemplified by the F100 engine manufactured by Pratt & Whitney Aircraft, a division of United Technologies Corporation, the assignee of this patent application.
Essentially, as can be seen from these graphs (FIG. 1), the thrust level varies from zero thrust (idle) to approximately 12,000 pounds of thrust (military power) at 0.9/15,000 ft. The area of the opening of the exhaust nozzle is pre-selected at 3.0 ft..sup.2 for idle and slightly smaller, 2.8 ft..sup.2 for intermediate power operation. These are optimum exhaust nozzle areas for steady state engine operation. Reference letter A on all the graphs represent the military power condition (12,000 pounds thrust) during normal operation of the aircraft. A typical snapdown in thrust desired would schedule the power to reduce to perhaps, say 2-4 thousand pounds of thrust or even idle (reference letter B) by reducing the amount of fuel to the combustor of the engine and increasing the exhaust nozzle area to 3.0 square feet. The N1 and N2 speeds, turbine temperature, and airflow will be at values in accordance with the thrust decreasing along the 3.0 ft..sup.2 Path, (E). For example, at idle, T4 would be at approximately 1200.degree. F. N1 and N2 speed would be at approximately 5000 RPM and 10,000 RPM, respectively and engine air flow would be at approximately 100 pounds/sec.
From the foregoing, it will be appreciated that under a typical schedule there is a significant decrease in speed, temperature and airflow in the engine when the power plant undergoes a transient from military power to idle power. Likewise, there is a significant increase in speed, temperature, and airflow when the powerplant undergoes a transient from idle or part power up to military power, just the reverse of the down power transient. Such scheduling is exemplary of fighter aircraft engines and represents a typical excursion of the internal engine performance characteristics.
What is also well known is the fact that the components of an engine designed for fighter type aircraft (fighter class) have a shorter life span than a similar part used in an engine designed to power commercial or non-fighter types of aircraft. Obviously, the severity of the thrust transients and the rapidity of such transients are more evidenced in the military engines than in non-military engines.
We have found that we can improve the power plant by including in addition to the heretofore known and above described schedule a different schedule utilized solely during predetermined transient conditions so as to afford the following benefits:
(1) improved life cycle fatigue (LCF) life, PA1 (2) improved engine stability/operability, PA1 (3) improved performance, and PA1 (4) reduced engine thrust transient time.
In preliminary analytical studies it has been determined that changing the scheduling of the power plant in accordance with this invention, there is a likelihood of attaining 1.4.times.and 2.7.times.F/M life benefits of the high and low rotors, respectively. Such predictions are based on assessment of the high physical rotor speed at idle on the surface flow fracture mechanics life for the engine evaluated.